4.不過(guò)當先行詞是all的時(shí)候,定語(yǔ)從句通常用that引導,如果all是主句主語(yǔ),那么all that兩個(gè)單詞可以用what代替,不過(guò)what引導的叫主語(yǔ)從句；如果all在主語(yǔ)中作賓語(yǔ),剛換成what剛引導的叫賓語(yǔ)從句。

　　5.但是that和what是不可能同時(shí)引導定語(yǔ)從句的。

　　如：All that he taught me was very useful.= What he taught me was very useful.前句中的that引導定語(yǔ)從句,all為先行詞,而后句中的what引導的叫主語(yǔ)從句,what在主語(yǔ)從句中作taught的直接賓語(yǔ)。